Shawn Dodds Thesis defense of Ph.D. , July 12, 2011

Stretching and Slipping Liquid Bridges: Liquid Transfer in Industrial Printing

Thesis defense of Ph.D. candidate Shawn Dodds
Department of Chemical Engineering and Materials Science

Advisors: Satish Kumar and Marcio Carvalho of IPrime’s Coating Process Fundamentals program.

Held Tuesday, July 12, 2011, 2:00 p.m., 3-180 Keller Hall, University of Minnesota.

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Liquid bridges with moving contact lines are found in a variety of
settings, such as capillary feeders and high-speed printing processes.
Despite this relevance, studies on liquid bridges often assume that
the contact lines remain pinned in place during stretching. While
this may be the case for some applications, contact line motion is
desirable in printing processes so that the amount of liquid
transferred can be maximized. In this thesis we study several model
problems to improve our understanding of how moving contact lines
alter the dynamics of liquid bridges.

We use the finite element method to study the stretching of a liquid
bridge between either two flat plates or a flat plate and a cavity.
For axisymmetric bridges we find that while the wettability of the two
surfaces is a key factor in controlling liquid transfer between two
flat plates, the presence of a cavity leads to fundamentally different
bridge dynamics. This is due to the pinning of the contact line on
the cavity wall, which leads to a decrease in the amount of liquid
transferred to the flat plate. We find that the presence of inertia
aids in cavity emptying by forcing the interface further into the
cavity. However, this increase in emptying can be offset by an
increased tendency for the production of satellite drops as the flat
plate is made more wettable.

To study non-axisymmetric flows we solve the Navier-Stokes equations
in three dimensions. We find that when the stretching motion is
asymmetric the liquid remains evenly distributed after breakup, so
long as the two plates are not accelerating relative to each other. If
the bridge shape is not initially cylindrical we find that the ability
of the bridge to maintain its initial shape after breakup depends on
the friction between the contact line and the solid.

Finally, we use flow visualization to observe the stretching of liquid
bridges both with and without small air bubbles. We find that while
the breakup of wetting fluids between two identical surfaces is
symmetric about the bridge midpoint, contact line pinning breaks this
symmetry at slow stretching speeds for nonwetting fluids. We exploit
this observation to force the bubbles selectively toward the least
hydrophillic plate confining the bridge.